This invention relates to nuclear reactor fuel elements, in particular to nuclear reactor fuel elements for pressurised water reactors (PWR'S). Such fuel elements comprise nuclear fuel material (usually pellets of uranium dioxide) enclosed in protective sheaths of zirconium alloy, the sheathed nuclear fuel material forming rods which are assembled into clusters in which the rods are arranged in a regular array or lattice.
It is current practice for helium gas to be included within these rods of sheathed nuclear fuel material. An initial purpose for its inclusion was to obivate the collapse of the sheaths into gaps created by sintering of pellets of the fuel material and, whilst this problem of excessive shrinkage of the pellets is now considered to have been largely overcome, the inclusion of helium still serves a useful purpose in delaying interaction between a sheath and the pellets it encloses. Further it is believed that, by its improvement in heat conductance across the gap between a sheath and its enclosed pellets, the release of fission product gases should be reduced and it may also assist in reducing other effects which are undesirable. However, during a reactor transient in which coolant pressure is lost in helium pressure, especially at low burn-ups, supplements the fission product gases in producing swelling of the sheath and the swelling may be so significant that sheath to sheath interaction occurs providing mutual support. Further deformation of the sheaths can then take place so that, at temperatures below which the swelling would be determined by local rupture of the sheath, blockage of coolant channels results. Furthermore the problem is not solved by a lower helium pressure in the rods when they are filled because, while these reduced pressures will decrease the swelling at the temperatures at which blockages would previously have occurred, there is some doubt onn extrapolation to higher temperatures which may be reached in a reactor transient. Local rupture may still not precede blockage since at the higher temperatures other factors may be introduced which affect the relevant properties of the sheath.